U.S. senators are debating whether to call witnesses in the impeachment trial of President Donald Trump, with a crucial make-or-break vote expected later on Friday. 

Two key Republican senators have indicated they do not plan to support the Democrats' push to call for more testimony. Lamar Alexander and Lisa Murkowski announced their opposition to seeking new evidence. They are two of four Republicans whom Democrats have been trying to persuade for the last two weeks. 

Without them, Democrats are unlikely to succeed in bringing forward witnesses like former National Security Advisor John Bolton.  

Republicans have repeatedly pushed back against the Democrats, arguing Bolton could publicly expose classified and national security information if he testified. 

A simple majority would be needed to allow witnesses, or 51 of the 100 senators. But with more Republicans than Democrats in the U.S. senate, the Democrats need to flip at least four rival senators.   

If witnesses are approved, the trial could last for several more weeks. 

President Trump is the third president in U.S. history to be impeached, but this could be the first trial without witnesses. 

If the call for witnesses is rejected, Republicans are hoping to move swiftly to a vote on the two articles of impeachment - abuse of power and obstruction of Congress.  

Ultimately, Trump is expected to be acquitted. At least 20 Republicans would need to vote to remove the president to achieve the two-thirds majority threshold of 67 votes. 

That vote to convict or acquit could come late Friday night or stretch into next week, depending on how long it takes to complete closing arguments.